As yummyfajitas said, this is just an example of how the actual issue manifests, but here's what that sentence means: Democrats and Republicans have their own

As yummyfajitas said, this is just an example of how the actual issue manifests, but here's what that sentence means: Democrats and Republicans have their own biases. These biases may be skew their thought processes and make them perform differently in mathematics tests which are worded differently. For example, a Republican may (unconciously) overshoot the numbers for a question about healthcare costs, or a Democrat for a question about military expenditure. Although this shouldn't happen, since mathematics tests are quite rigorously worded, it might, and a researcher is interested in investigating further.